1. Field Of The Invention
The present invention is directed to signal multiplexers. More specifically, the present invention is directed to a signal multiplexer circuit having a multiplexed signal reference level equalizer.
2. Description Of The Prior Art
The multiplexing of an input signal to a contemporary digital computer process control system involves a selection of a process signal from a plurality of remotely generated signals for application to a digital computer interface such as an analog-to-digital converter. In order to ascertain the operability of the interface, a known calibration signal is provided for selection according to a predetermined sequence. The calibration signal, thus, provides a known reference against which the operation of the interface can be checked. On the other hand, the calibration signal is usually generated locally while the process signals are remote, or field, generated signals which gives rise to a difference in signal common, or reference, levels. In order to allow the interface to accept either type of input signals, the local and remote signals must be equalized, i.e., the offset of their reference levels is compensated for, to allow either type of input signal to be utilized by the process control system.